Heisenberg picture operators in the stochastic wave function approach to open quantum systems

A fast simulation algorithm for the calculation of multitime correlation functions of open quantum systems is presented. It is demonstrated that any stochastic process which ``unravels'' the quantum Master equation can be used for the calculation of matrix elements of reduced Heisenberg picture operators, and thus for the calculation of multitime correlation functions, by extending the stochastic process to a doubled Hilbert space. The numerical performance of the stochastic simulation algorithm is investigated by means of a standard example.Comment: RevTex, 5 pages, 2 figures, uses multico

Non-Markovian barrier crossing with two-time-scale memory is dominated by the faster memory component

We investigate non-Markovian barrier-crossing kinetics of a massive particle in one dimension in the presence of a memory function that is the sum of two exponentials with different memory times τ 1 and τ 2 . Our Langevin simulations for the special case where both exponentials contribute equally to the total friction show that the barrier crossing time becomes independent of the longer memory time if at least one of the two memory times is larger than the intrinsic diffusion time. When we associate memory effects with coupled degrees of freedom that are orthogonal to a one-dimensional reaction coordinate, this counterintuitive result shows that the faster orthogonal degrees of freedom dominate barrier-crossing kinetics in the non-Markovian limit and that the slower orthogonal degrees become negligible, quite contrary to the standard time-scale separation assumption and with important consequences for the proper setup of coarse-graining procedures in the non-Markovian case. By asymptotic matching and symmetry arguments, we construct a crossover formula for the barrier crossing time that is valid for general multi-exponential memory kernels. This formula can be used to estimate barrier-crossing times for general memory functions for high friction, i.e. in the overdamped regime, as well as for low friction, i.e. in the inertial regime. Typical examples where our results are important include protein folding in the high-friction limit and chemical reactions such as proton-transfer reactions in the low-friction limit

Charge carrier transfer in the gas electron multiplier at low gas gains

Connected to the Linear Collider project TESLA at DESY, studies on the readout of TPCs based on the GEM-technology are ongoing. For particle identication via dE/dx - measurement, a good energy resolution is indispensable, and therefore losses of primary electrons have to be avoided. It turned out, that in the GEM transverse diffusion inside or close to the holes is a not negligible reason for these losses. For Ar-CH4 90:10 and TPC-like field configurations it was found, that when operated in normal amplification mode, the Standard Geometry GEM should not lose primaries, whereas for low gains, also when operated in magnetic fields up to 5T, a GEM with larger pitch and hole diameter would be necessary

Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria

Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)_(aq) and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in ^(56)Fe/^(54)Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ^(56)Fe_(Fe(OH)3–Fe(II)aq) fractionation factor of +3.0‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)_(aq) and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)_(aq) by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)_(aq) oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)_(aq) and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation

Aging measurements with the gas electron multiplier (GEM)

Continuing previous aging measurements with detectors based on the Gas Electron Multiplier (GEM), a $31\times 31$cm$^2$ triple-GEM detector, as used in the small area tracking of the COMPASS experiment at CERN, was investigated. With a detector identical to those installed in the experiment, long-term, high-rate exposures to $8.9$keV X-ray radiation were performed to study its aging properties. In standard operation conditions, with Ar:CO$_2$ (70:30) filling and operated at an effective gain of $8.5\cdot 10^3$, no change in gain and energy resolution is observed after collecting a total charge of 7mC/mm$^2$, corresponding to seven years of normal operation. This observation confirms previous results demonstrating the relative insensitivity of GEM detectors to aging, even when manufactured with common materials